1. Field of the Invention
The present invention relates to area-efficient reconstruction filters, particularly for current-driven digital-to-analog converters (DAC).
2. Discussion of the Related Art
Digital-to-analog converters are conventionally used very frequently in integrated circuits.
Since these converters are sampled-data circuits, in addition to generating the intended analog signal in the correct frequency range or base band, they also produce an undesirable duplicate image of the signal, generally designated as "imaging", as shown in FIG. 1, which plots the output of the DAC as a function of the frequency f.
The chart shows that in addition to the output signal, designated by S (where B is the base band), there is also a duplicate image of the signal S which is centered around the sampling frequency f.sub.s of the DAC.
In order to eliminate this duplicate image, a continuous-time low-pass reconstruction filter is usually introduced and placed downstream of the DAC, as shown in FIG. 2.
In this figure, the reference numeral 1 designates an N-bit DAC, where b.sub.0, b.sub.1, . . . b.sub.n-1 are the input bits of the DAC and V.sub.DAC and I.sub.DAC are, respectively, the output voltage and the output current of the DAC; the :reference numeral 2 instead designates a continuous-time low-pass reconstruction filter arranged downstream of the DAC; V.sub.o is the output voltage.
The reconstruction filter 2 must provide high attenuation for frequencies close to the sampling frequency f.sub.s of the DAC, but at the same time it must be efficient in terms of area occupation if the DAC is to be used in an integrated circuit, where of course the requirement of minimum area occupation is one of the most important factors.
It is known to those skilled in the art that these are two mutually contrasting requirements.
It is therefore necessary to achieve a compromise, shown in FIGS. 3a and 3b. The filters shown in these figures are second-order low-pass filters. The solution shown in FIG. 3b, however, is the one that is practically mandatory when working with supply voltages of less than 3V.
This solution is rather area-efficient when the input signal is a voltage, but it is highly insufficient when the input signal must be a current, as shown in FIG. 3c by applying only the Norton equivalent to the input of FIG. 3b.
A numeric example is now described to clarify the above explanation.
Assume that a DAC has been devised which has a full-scale voltage output V.sub.iFS and that a full-scale voltage from the reconstruction filter is required as output. Assume also that a cutoff frequency of approximately 270 kHz is chosen for the filter.
The values of the components of FIG. 3b will be as follows: EQU R.sub.1 =R.sub.2 =R.sub.3 =50 kohm C.sub.1 =25.2 pF C.sub.2 =5.6 pF
Assume also that one intends to use a DAC with a full-scale current output I.sub.DACFS =160 .mu.A and that one seeks a full-scale output voltage V.sub.OFS =0.5 V from the reconstruction filter with a frequency response which is identical to that of the filter used previously with the voltage-output DAC.
Since one must have R.sub.3 I.sub.DACPS =V.sub.OFS, then R.sub.3 =3.125 kohm and therefore R.sub.1 =R.sub.2 =R.sub.3 =3.125 kohm.
Therefore, in order to have the same frequency response as the preceding filter, the values of C.sub.1 and C.sub.2 must be 403.2 pF and 89.6 pF respectively.
Accordingly, the area occupied on the silicon in order to integrate the values of these components is approximately sixteen times greater than the area occupied to integrate the components of FIG. 3b, due to the relatively low specific capacitance that can be provide in integrated circuits.
Therefore, the reconstruction filter, in the case of a current input (and therefore of a current output of the DAC), is very wasteful from the point of view of the area occupied on the silicon wafer.